1. Field of the Invention
The present invention relates to golf balls. Specifically, the present invention relates to multi-piece golf balls that include a center, an envelope layer, a mid layer, and a cover.
2. Description of the Related Art
Golf players' foremost requirement for golf balls is flight performance. Golf players place importance on flight performance upon shots with a driver and a long iron. Flight performance correlates with the resilience performance of a golf ball. When a golf ball with excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.
An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. In a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. In a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of a core having an outer-hard/inner-soft structure can achieve a low spin rate and a high launch angle.
In light of achieving various performance characteristics, golf balls each having a multilayer structure have been proposed. JPH9-56848 (U.S. Pat. No. 5,725,442) discloses a golf ball that includes a core, an inner cover, and an outer cover. The core consists of an inner sphere and an envelope layer.
JPH10-328326 (U.S. Pat. No. 6,468,169) discloses a golf ball that includes a core, an envelope layer, an inner cover, and an outer cover.
JP2001-17575 (U.S. Pat. No. 6,271,296) discloses a golf ball that includes a core, an envelope layer, a mid layer, and a cover.
JP2002-272880 (U.S. Pat. No. 6,913,547) discloses a golf ball that includes a core and a cover. The core consists of a center and an outer core layer. The cover consists of an inner cover layer and an outer cover layer.
JP2003-205052 (US 2003/0166422) discloses a golf ball that includes a center, a mid layer, and a cover.
JP2004-130072 (US 2004/0029648) discloses a golf ball that includes a core and a cover. The core has a three-layer structure.
JP2006-289065 (US 2006/0229143 and US 2007/0155541) discloses a golf ball that includes a core and a cover. The core consists of an inner layer and an outer layer.
A shot with a middle iron is mainly intended to carry a golf ball onto a putting green. Even for a shot with a middle iron, a golf ball having a large flight distance is advantageous similarly to shots with a driver and a long iron.
The loft angle of a middle iron is greater than that of a long iron. Upon a shot with a middle iron, the spin rate is high as compared to that upon a shot with a long iron. The trajectory upon a shot with a middle iron is high. The trajectory upon a shot with a middle iron is likely to be influenced by wind. In particular, when a headwind blows, a flight distance is considerably impaired. A golf ball that is unlikely to be influenced by wind upon a shot with a middle iron is desired.
Golf players also place importance on spin performance of golf balls. When a backspin rate is high, the run is short. It is easy for golf players to cause a golf ball, to which backspin is easily provided, to stop at a target point. When a sidespin rate is high, the golf ball tends to curve. It is easy for golf players to intentionally cause a golf ball, to which sidespin is easily provided, to curve. A golf ball to which spin is easily provided has excellent controllability. In particular, advanced golf players place importance on controllability upon a shot with a short iron.
An object of the present invention is to provide a golf ball having excellent flight performance when being hit with a middle iron. Another object of the present invention is to provide a golf ball having excellent controllability when being hit with a short iron.